Some people, for a variety of reasons, claim to have found faults with the hypothesis that humans are affecting Earth's climate. While we strive to present any legitimate criticism of the scientific principles where they are presented, this section includes some specific issues that are commonly cited as reasons that humans are not or could not change the climate.

Lack of scientific consensus

One of the most common arguments against human induced climate change is a supposed lack of scientific consensus. While there were many skeptical scientists in the past, as the evidence has mounted (especially using satellite-based data), even the most ardent skeptics have come to the determination that humans are changing the climate. One recent study found no instances in the peer-reviewed literature of a study on climate change stating that global warming is either fictitious or purely natural Oreskes, N., Science, 2004, 306(5702), DOI: 10.1126/science.1103618. See also Scientific Consensus.

The Earth's surface obtains energy from four primary natural sources: space (predominantly solar radiation), the molten core of the Earth, anthropogenic processes that generate excess heat, and radiation from the atmosphere. The second (geothermal heat) is known to be trivially small; the third (direct excess heat) is not as important as increases in the fourth, the energy retention (called the "greenhouse effect"), that is the property of retention heat that could be lost to space due to changes in the atmosphere or surface characteristics, this retention today is mostly due to increases in CO2 levels and receding reflective surfaces like ice or snow. So while indications seems to point to human activities and it is nice to think that changing our energy consumption habits will stop global warming, consideration is needed to account that climate change can be driven by processes that we may have little control over.

Note:
Solar radiation normally varies over time as the orbit of the earth changes due to gravitational inter-action with the other planets and the sun. When the Earth's orbit gets more elliptical and most of the land mass is in a seasonal orbital position so it is receiving more direct radiant energy from the sun (or it is summer for most of the land mass when the orbit brings it nearest the sun), then an inter-glacial period usually occurs. Those combining planetary science with geologic evidence have significant findings suggesting that our present inter-glacial period may have not peaked. Some pointing to an inter-glacial period about 400,000 years ago that had about 1/3 of the ice on the Antarctic gone when it peaked, as having the most similar pattern of orbits for the planets when compared to the orbits now.

Also note that the current configuration actually has Earth closest to the sun during northern hemisphere winter, and not summer. Seasons are not due to the eccentricity (how "oval" the orbit is), but really much more on the tilt of the spin axis of Earth (obliquity). There is a precession signal as well, which is influenced by the sun-earth distance, but that signal is more directly linked to the Tropics.

Others, like Bill Ruddiman (U VA) think that we are overdue for an ice age, based on orbital parameters.